Mixing RAM sticks with different frequencies

I've currently got 3 GB of RAM running at 400MHz, but I just picked up a 667MHz stick.
I've been reading online that mixing different frequencies isn't exactly the best move.
What's your take on this?

Are you talking about DDR400 for those 3GB? Because there's no way that 667MHz is part of the same set—that's definitely DDR2, and they won't play nice together.
You can't just mix different generations like DDR, DDR2, DDR3, or DDR4... it's a no-go.
Technically, you can run different sticks if the motherboard's BIOS recognizes them, but it'll just throttle everything down to match whatever the slowest stick in the bunch is.
Obviously, life is much easier and smoother if you just use identical sticks from the jump.

If you're looking at CPU-Z, those 400MHz figures actually represent an 800MHz DDR2 clock speed (since DDR stands for double data rate), which explains why half the info in CPU-Z looks
weird.
Generally speaking, you can mix and match different RAM speeds—provided they are the same type—but the system will simply default to the speed of the slowest stick. Honestly, you won't even notice the performance difference. Alternatively, you could try bumping up the frequency of that slower module by one notch, though I'm not sure which specific sticks might resist that.

That said, there is always a chance that certain modules just refuse to play nice together. You can't predict that based on specs alone; it’s purely a matter of whether the hardware decides to be difficult or not.

All 4 GB are DDR400. Forgot to mention that. Only the 3 GB I already have is running at 400, while the stick I just picked up is rated for 667.

It’ll work just fine. It's just gonna be stuck running at a pathetic 400 MHz memory speed, that's all.